The present invention relates to methods for manufacturing Rxe2x80x94Fexe2x80x94B type rare earth magnets, alloy powder for such magnets, and magnets produced by such methods.
Rare earth sintered magnets are produced by pulverizing an alloy for rare earth magnets to form alloy powder, compacting the alloy powder, and subjecting the alloy powder to sintering and aging. Presently, as the rare earth sintered magnets, two types of magnets, that is, samarium-cobalt magnets and neodymium-iron-boron magnets, are extensively used in various fields. In particular, neodymium-iron-boron magnets (hereinafter, referred to as xe2x80x9cRxe2x80x94Fexe2x80x94B type magnetsxe2x80x9d, where R is any rare earth element and/or Y, Fe is iron, and B is boron), which exhibit the highest magnetic energy product among a variety of magnets and have a comparatively low cost, have been vigorously applied to various types of electronic equipment. Note that a transition metal element such as Co may substitute for part of Fe and C may substitute for part of B.
Powder of the material alloy for Rxe2x80x94Fexe2x80x94B type rare earth magnets may be produced by a method including a first pulverization process for coarsely pulverizing the material alloy and a second pulverization process for finely pulverizing the material alloy. In general, in the first pulverization process, the material alloy is coarsely pulverized to a size of the order of several hundred micrometers or less using a hydrogen embrittlement apparatus. In the second pulverization process, the coarsely pulverized alloy (coarsely pulverized powder) is finely pulverized to an average particle size of the order of several micrometers with a jet mill or the like.
The material alloy can be produced by methods largely classified into two types. The first type is an ingot casting method where a molten alloy is poured into a mold and cooled comparatively slowly. The second type is a rapid cooling method, typified by a strip casting method and a centrifugal casting method, where a molten material alloy is put into contact with a single chill roll, twin chill rolls, a rotary chill disk, a rotary cylindrical chill mold, or the like, to be rapidly cooled thereby producing a solidified alloy thinner than an ingot cast alloy.
In the rapid cooling method, the molten alloy is cooled at a rate in the range between 1022xc2x0 C./sec and 104xc2x0 C./sec. The resultant alloy produced by the rapid cooling method has a thickness in the range between 0.03 mm and 10 mm. The molten alloy starts solidifying at the face that comes into contact with a chill roll. From the roll contact face, crystal grows in the thickness direction into the shape of pillars or needles. The resultant rapidly solidified alloy therefore has a fine crystal structure including portions of a R2T14B crystal phase having a size in the range between 0.1 xcexcm and 100 xcexcm in the minor-axis direction and in the range between 5 xcexcm and 500 xcexcm in the major-axis direction, and portions of an R-rich phase dispersed at grain boundaries of the R2T14B crystal phase portions. The R-rich phase is a nonmagnetic phase in which the concentration of any rare earth element R is relatively high, and has a thickness (which corresponds to the width of the grain boundaries) of 10 xcexcm or less.
Because the rapidly solidified alloy is cooled in a relatively short time compared with an ingot alloy produced by a conventional ingot casting method, the alloy has a fine structure and is small in grain size. In addition, with finely dispersed crystal grains, the area of grain boundaries is wide, and thus the R-rich phase spreads thinly over the grain boundaries. This results in good dispersion of the R-rich phase.
When a rare earth alloy (especially a rapidly solidified alloy) is coarsely pulverized in a hydrogen embrittlement process where the rare earth alloy first occludes hydrogen (this way of pulverization is herein called xe2x80x9chydrogen pulverizationxe2x80x9d), the R-rich phase portions existing at grain boundaries react with hydrogen and expand. This tends to cause the alloy to crack from the R-rich phase portions (grain boundary portions). Therefore, the R-rich phase tends to be exposed on the surfaces of particles of the rare earth alloy powder obtained by the hydrogen pulverization. In addition, in the case of a rapidly solidified alloy, where the R-rich phase portions are fine and highly dispersed, the R-rich phase particularly tends to be exposed on the surfaces of the hydrogen-pulverized powder.
According to experiments performed by the present inventors, when the coarsely pulverized powder in the above state is finely pulverized by a jet mill or the like, R-rich super-fine powder (fine powder having a particle size of 1 xcexcm or less) is produced. Such R-rich super-fine powder particles oxidize very easily compared with other powder particles (having a relatively large particle size) that contain a relatively smaller amount of R. Therefore, if a sintered magnet is produced from the resultant finely pulverized powder without removing the R-rich super-fine powder, oxidation of the rare earth element vigorously proceeds during the manufacturing process steps. The rare earth element R is thus consumed for reacting with oxygen, and as a result, the production amount of the R2T14B crystal phase as the major phase decreases. This results in reducing the coercive force and remanent flux density of the resultant magnet and deteriorating the squareness of the demagnetization curve, which is the second quadrant curve of the hysteresis loop.
In order to prevent oxidation of the R-rich finely pulverized powder, the entire process from pulverizing through sintering may ideally be performed in an inert atmosphere. It is however very difficult to realize this in a mass-production scale in production facilities.
There is proposed a method for solving the above problem, where fine pulverization is performed in an inert atmosphere containing a trace amount of oxygen, to intentionally coat the surfaces of finely pulverized powder particles with a thin oxide film to thereby suppress fast oxidation of the powder particles in the atmosphere.
However, the present inventors have found that the above method fails to sufficiently improve the final magnet properties and maintain the properties at the highest level, as long as the finely pulverized powder contains R-rich super-fine powder in a percentage equal to or more than a predetermined value.
An object of the present invention is to provide alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets capable of sufficiently improving and stabilizing the magnet properties.
Another object of the present invention is to provide alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets capable of sufficiently improving the final magnet properties and maintaining the properties at the highest level even when a material alloy including an R-rich phase is used and such a material alloy is coarsely pulverized by the hydrogen pulverization method.
The method for manufacturing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets of the present invention includes a first pulverization step of coarsely pulverizing a material alloy for rare earth magnets and a second pulverization step of finely pulverizing the material alloy, wherein the first pulverization step comprises a step of pulverizing the material alloy by a hydrogen pulverization method, and the second pulverization step comprises a step of removing at least part of fine powder having a particle size of 1.0 xcexcm or less to adjust the particle quantity of the fine powder having a particle size of 1.0 xcexcm or less to 10% or less of the particle quantity of the entire powder.
In a preferred embodiment, the average concentration of the rare earth element contained in the fine powder having a particle size of 1.0 xcexcm or less is greater than the average concentration of the rare earth element contained in the entire powder.
Alternatively, the method for manufacturing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets of the present invention includes a first pulverization step of coarsely pulverizing a material alloy for rare earth magnets produced by a rapid cooling method and a second pulverization step of finely pulverizing the material alloy, wherein the second pulverization step comprises a step of removing at least part of powder in which the concentration of the rare earth element is greater than the average concentration of the rare earth element contained in the entire powder, to reduce the average concentration of oxygen bound with the rare earth element contained in the powder.
In the second pulverization step, the alloy is preferably finely pulverized using a high-speed flow of a gas.
Preferably, a predetermined amount of oxygen is contained in the gas. In this case, the concentration of the oxygen is preferably adjusted to be in the range between 0.05% and 3% by volume.
Plural types of rare earth alloys different in rare earth content may be used as the material alloy for rare earth magnets.
In an embodiment, the first pulverization step is performed separately for the plural types of rare earth alloys different in rare earth content, and the second pulverization step is performed one time together for the plural types of rare earth alloys different in rare earth content.
In another embodiment, the first and second pulverization steps are performed separately for the plural types of rare earth alloys different in rare earth content, and after the second pulverization step, the plural types of rare earth alloy powder are mixed together.
The alloys may be finely pulverized using a jet mill.
In a preferred embodiment, a classifier is provided following the jet mill for classifying powder output from the jet mill.
In a preferred embodiment, the material alloy for rare earth magnets is obtained by cooling a molten material alloy at a cooling rate in a range between 102xc2x0 C./sec and 104xc2x0 C./sec.
The molten material alloy is preferably cooled by a strip casting method.
In a preferred embodiment, the average particle size of the powder obtained in the first pulverization step is 200 to 1000 xcexcm. When the material alloy for rare earth magnets is produced by a rapid cooling method, the average particle size of the powder is typically 500 xcexcm or less.
The average particle size of the powder obtained in the second pulverization step is preferably in a range between 2 xcexcm and 10 xcexcm.
Preferably, the method further includes the step of adding a lubricant to the powder obtained in the second pulverization step.
The method for manufacturing an Rxe2x80x94Fexe2x80x94B type rare earth magnet of the present invention includes the steps of: preparing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets produced by any of the methods for manufacturing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets described above; and compacting the alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets to produce a permanent magnet.
Alternatively, the method for manufacturing an Rxe2x80x94Fexe2x80x94B type rare earth magnet of the present invention includes the steps of: preparing first alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets produced by any of the methods for manufacturing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets described above; preparing second alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets different from the first alloy powder in rare earth content; forming mixed powder by mixing the first alloy powder and the second alloy powder; compacting the mixed powder to produce a compact; and sintering the compact to produce a permanent magnet.
The alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets of the present invention has an average particle size in a range of 2 xcexcm and 10 xcexcm, and the particle quantity of fine powder having a particle size of 1.0 xcexcm or less is adjusted to 10% or less of the particle quantity of the entire powder.
In a preferred embodiment, the alloy powder is obtained by cooling a molten material alloy at a cooling rate in a range between 102xc2x0 C./sec and 104xc2x0 C./sec and pulverizing the resultant alloy.
The Rxe2x80x94Fexe2x80x94B type rare earth magnet of the present invention is produced from the alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets described above.